Orthogonal Frequency Division Multiplexing (OFDM) radio systems are currently under discussion in many places as e.g. in 3GPP Technical Specification Group (TSG) Radio Access Network (RAN1). Such a radio system should be a so-called frequency reuse of 1 network, i.e. a network in which all base stations use the same subcarrier frequencies, as W-CDMA (W-CDMA=Wideband Code Division Multiple Access) currently is.
Given the licensed bandwidth, transmission capacity from network providers e.g. for WEB surfing or video streaming or video communication has to be as high as possible for all users to serve as many subscribers as possible. Further the quality of service experienced by the user and especially the coverage of the service is an important property demanded by the user. So Orthogonal Frequency Division Multiplexing shall work well at the cell borders of a frequency reuse of 1 network and exploit the channel capacity.
OFDM is a multi-carrier modulation technique. The data is divided among a large number of closely spaced subcarriers. Several bits of a bit stream are mapped on one subcarrier by modulating the complex amplitude c by e.g. QPSK (QPSK=Quadrature Phase Shift Keying), 16-QAM or 64-QAM (QAM=Quadrature Amplitude Modulation). If the subcarriers that are spaced in frequency at intervals of 1/Tsym are emitted for a symbol duration Tsym, the resulting signals are all orthogonal giving the method its name.
In practice, the signals are not generated by oscillators but since we are working with sampled signals anyhow, the symbol is generated by an N point IFFT (IFFT=Inverse Fast Fourier Transformation). So with the sampling period T0, the symbol duration is N*T0=Tsym. The N complex amplitudes per subcarrier serve as the spectral coefficients S(k). Since k serves as the frequency index this can also be denoted S(f) with f denoting the frequency.
In cellular systems with a frequency reuse factor of 1 the signal to interference ratio at the cell border approaches the factor 1 or 0 dB, so that no useful transmission from the base station to the mobile terminal can be kept up.
Therefore in CDMA transmission soft handover was introduced. Also both base stations use a differently scrambled pilot signal.
Single carrier frequency division multiplexing (FDM) also provides the frequency selective properties of OFDM.
In OFDM or single carrier FDM transmission, frequency groups are allocated to a mobile terminal instead of codes in CDMA transmission. Cross-cell interference can be coordinated allowing improvement of reception in the interference region. OFDM offers the possibility to flexibly allocate one or more subcarriers to one user or one logical channel, respectively, to control the data rate for this user channel.
Further it is assumed that channel estimation in the interference region of two cells is possible. This can e.g. be achieved by pilot symbols having higher energy than the data symbols which allows channel estimation even if the pilot symbol of the serving cell falls on the same subcarrier as the data symbol of the interfering cell. Additionally it is necessary that the pilot symbols of different cells do not fall on the same subcarrier, so the pilots are distributed on different frequency grids in different cells to allow channel estimation in the interference region. Alternatively the channel estimation in the interference region can be achieved by pilot symbols that use a spreading code and additionally a scrambling code that is cell specific and different for neighbor cells. Then the pilot symbol sequence is despreaded before used for channel estimation.
Adaptive subcarrier allocation or frequency scheduling is known to improve the transmission capacity of an OFDM or FDM transmission system. Due to different channel transfer functions and different interference for each mobile terminal, subcarriers or parts of the spectrum are better or worse suited for transmission dependent on the mobile terminal. This is indicated by the signal to interference ratio function of each mobile terminal. So the approach is to select, in the base station, the best subcarriers for each mobile terminal and allocate them to the respective mobile terminal in order to maximize the cell throughput. This shall also be done at the cell border where one or more dominant interferers exist and the interference cannot be assumed white anymore.
A method for an adaptive subcarrier allocation is described in “Multiuser OFDM with Adaptive Subcarrier, Bit, and Power Allocation” by Wong et al., IEEE Journal On Selected Areas In Communications, vol. 17, no. 10, October 1999. There, it is described that the base station estimates channel characteristics based on received uplink transmissions.